GN: default to -O1

In a GN world we are no longer tied to the Debug/Release dichotomy of
GYP, and extra_cflags makes it trivially easy to override any flags we
set by default.  These two together mean we don't have to make the default
Debug build pointlessly unoptimized.

-O1 takes negligibly longer to compile than -O0, doesn't make debugging
difficult, and the code is considerably better.  DM runs ~4x faster this way.

Backstory: I just re-noticed we do this for our Debug MSAN builds, which has
had a serious wall-time slimming effect.  I was about to restore it to my GN
MSAN builds when I realized, hey, maybe we should do this for all builds.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2322753002

Review-Url: https://codereview.chromium.org/2322753002

gn/BUILD.gn

@@ -28,6 +28,7 @@ config("no_rtti") {
 
 config("default") {
   cflags = [
+    "-O1",
     "-g",
     "-fstrict-aliasing",
     "-fPIC",

src/core/SkGlyphCache.cpp

@@ -545,7 +545,7 @@ SkGlyphCache* SkGlyphCache::VisitCache(SkTypeface* typeface,
 
     // Precondition: the typeface id must be the fFontID in the descriptor
     SkDEBUGCODE(
-        uint32_t length;
+        uint32_t length = 0;
         const SkScalerContext::Rec* rec = static_cast<const SkScalerContext::Rec*>(
             desc->findEntry(kRec_SkDescriptorTag, &length));
         SkASSERT(rec);